A problem with zonotopic implementations of
REF  is that zonotopes are not closed under
convex hulls, and so the sets \(\Gamma _i\)  defined in
REF  are not, in general, zonotopes.
We here follow an idea by Girard [1]} and
replace \(\Gamma _i\)  by a zonotope obtained using the enclosure operator
\(\colon \left( \mathbb {R}^n \times \mathbb {R}^{n \times p} \right)^2\rightarrow \mathbb {R}^n \times \mathbb {R}^{n \times (2 p + 1)}\) 
given by
\(((b,F),(c,G))=\left(\frac{b+c}{2},\left(\frac{F+G}{2},\frac{b-c}{2},\frac{F-G}{2}\right)\right)\) 
